$\displaystyle \begin{array}{{>{\displaystyle}l}} \sqrt{x-\sqrt{x-1}} =3\\ DK\ :x\geqslant 1\ \\ \rightarrow x-\sqrt{x-1} =9\ \\ \rightarrow x-9=\sqrt{x-1}\\ \rightarrow ( x-9)^{2} =x-1\\ \rightarrow x^{2} -18x+81-x+1=0\ \\ \rightarrow x^{2} -19x+82=0\ \\ sử\ dụng\ delta\ tìm\ được\ \left[ \begin{array}{l l} x_{1} =\frac{19+\sqrt{33}}{2} >0( tm) & \\ x_{2} =\frac{19-\sqrt{33}}{2} < 0 & \end{array} \right.\\ Vậy\ tập\ nghiệm\ pt\ là\ S=\left\{\frac{19+\sqrt{33}}{2}\right\} \end{array}$
